The present invention relates to electromagnetic electro-acoustic transducers for generating an incoming indicator tone such as in mobile phones.
Referring to FIG. 5, a description will be given on a conventional electromagnetic electro-acoustic transducer. FIG. 5 (a) is a top view and FIG. 5 (b) is a cross-sectional view.
The conventional electromagnetic electro-acoustic transducer comprises a first diaphragm 100, a second diaphragm 101 being a magnetic material attached on the center of the first diaphragm 100, a center pole 103 disposed opposite the second diaphragm 101, a coil 104 wound around the center pole 103, a ring-shaped resin magnet 105 positioned on the outer periphery of the coil 104, a yoke 106 in contact with or integrated with the center pole 103, and a cylindrical housing 107 that circumferentially supports the first diaphragm 100. A description will be given on the operation of an electromagnetic electro-acoustic transducer of the above configuration. In the initial state when no current is flowing in the coil 104, a magnetic circuit is formed by the resin magnet 105, second diaphragm 101, center pole 103, and yoke 106, and the second diaphragm 101 is attracted toward the resin magnet 105 and the center pole 103. Thereby the first diaphragm 100 is displaced to a position that balances with its elastic force.
Next, when an alternating current flows in the coil 104, an alternating magnetic field is generated by the magnetic circuit where the coil 104 works as a magnetomotive force. The magnetic flux density within the magnetic circuit is determined by the intensity of the alternating magnetic field and the magnetic resistance inside the magnetic circuit. In this case, the magnetic resistance is approximately equal to the combined resistance of the magnetic resistance due to the magnetic gap between the second diaphragm 101 and the center pole 103, the magnetic resistance due to a magnetic gap between the second diaphragm 101 and the resin magnet 105, and the magnetic resistance of the resin magnet 105 itself. The specific magnetic permeability of the resin magnet 105 is as low as that of the air and is approximately equal to 1, so that the magnetic resistance is high.
An alternating driving force is generated on the second diaphragm 101 due to a change in the magnetic flux density. As a result, the second diaphragm 101 moves from the initial position together with the attached first diaphragm 100 due to a static attraction force generated by the resin magnet 105 and a change in the alternating driving force generated by an alternating current. And its vibration radiates a sound.
The above-mentioned resin magnet 105 is a composite material consisting of a hard ferrite magnetic material, a polyamide resin such as nylon 6, nylon 12, and low molecular weight rubber. That is, the resin magnet 105 in the conventional transducer uses a composite material of a hard magnetic material and a resin.
By the way, as electromagnetic electro-acoustic transducers are used in mobile phones and the like, their small size and a high sound pressure are required for their alternating driving force. Accordingly, the acuteness of resonance, Q-factor of the mechanical resonant system consisting of the first diaphragm 100 and the second diaphragm 101 is made high, and the lowest resonant frequency of the mechanical resonant system are made close to the regeneration acoustic frequency.
Also, while the lowest resonant frequency of the mechanical resonant system is determined by the effective masses of the first diaphragm 100 and the second diaphragm 101 and the stiffness of the first diaphragm 100, the stiffness of the first diaphragm 100 is not only affected by the modulus of elasticity and thickness of the material used but also by the configuration of deformation due to the static attraction force by the resin magnet 105 and the center pole 103.
The conventional electromagnetic electro-acoustic transducer as described above suffered a problem of resolving the complicated situation as described below in order to obtain an electromagnetic electro-acoustic transducer with a desired characteristic.
(1) In order to change the lowest resonant frequency of the mechanical system depending on the regenerated acoustic frequency, it is necessary to change the effective mass and stiffness of the mechanical resonant system. In doing this, it is necessary to change the thickness of the first diaphragm 100 and the thickness and diameter of the second diaphragm 101, or to change the magnetic gap between the second diaphragm 101 and the center pole 103 in order to change the static attraction force. Such changes in the thickness and diameter of the second diaphragm 101 will result in changes in the magnetic resistance and in various interacting mechanical parameters including effective mass and vibration mode the regeneration acoustic frequency. Consequently such a design change to get a desired characteristic become complicated.
(2) As the acuteness of resonance, Q-factor of the mechanical resonant system is high, the ratio of contribution of the lowest resonant frequency to sound pressure is extremely high, making sound pressure variation due to a slight change in a magnetic gap or in the thickness of the diaphragm large.
However, in order to make the acuteness of resonance, Q-factor low, although lightening of the mass of the vibratory system or an increase in the driving force (force coefficient) is necessary, lightening of the mass of the vibratory system makes the volume of the second diaphragm 101, being a magnetic material, small and will result in an increase in the magnetic saturation and magnetic resistance thus resulting in a decrease in the force coefficient.
Also, in order to increase the force coefficient, it is necessary to increase either direct current magnetic flux density or alternating current magnetic flux density; and in order to increase the direct current magnetic flux, it is necessary to increase energy of the magnet or upsize the diaphragms thus leading to upsizing of the total size or an increase in the mass of the vibratory system.
Although it is necessary to decrease the magnetic resistance in order to increase the alternating current magnetic flux, it is difficult as the magnetic permeability of conventional resin magnet 105 is low. There is other approach such as making the magnetic gaps small but it makes vibration amplitude of the diaphragms small.
The present invention resolves the above-described problems and provides an electromagnetic electro-acoustic transducer in which the cost is low, the lowest resonant frequency of the mechanical system is variable, and variation of sound pressure at high sound pressures is narrow.
A first embodiment of the present invention comprises a first diaphragm, a second diaphragm attached at the center of the first diaphragm and consisting of a magnetic material smaller than the first diaphragm, a center pole provided underneath the center 6f the second diaphragm with a magnetic gap in between, a coil wound around the outer periphery of the center pole, a ring-shaped resin magnet, and a yoke disposed in a manner such that it comes in contact with the lower parts of the center pole. Here, the magnetic powder of said resin magnet consists of powders of a hard magnetic material and a soft magnetic material, the magnetic flux density and the magnetic permeability can be controlled by changing their compounding ratio, so that lowest resonant frequency and the high sound pressure can be set very easily.
A second embodiment of the present invention is one in which the magnetic powder orientation of the resin magnet of the first embodiment is aligned by injection molding in magnetic field. Magnetic energy is enhanced when magnetizing the hard magnetic material in the resin magnet, -thereby obtaining a resin magnet with a high magnetic flux density and a high magnetic permeability jointly by mixing of a soft magnetic material.
A third embodiment of the present invention is one in which the hard magnetic material in the resin magnet of the first embodiment is a eutectic composition of a ferrite magnetic material and a rare-earth magnetic material. By employing a eutectic composition of a ferrite and a rare-earth magnetic material as the resin magnet, a higher magnetic flux density is obtained thereby enabling provision of a further superior electromagnetic electro-acoustic transducer.
A fourth embodiment of the present invention is one in which the total amount of the magnetic powder within the resin magnet of the first embodiment is in the range 85 to 92% by weight thus enabling fabrication of a resin magnet with superior moldability and magnetic characteristic thereby providing a superior electromagnetic electro-acoustic transducer
A fifth embodiment of the present invention is one in which a ring-shaped magnetic plate with an inner diameter smaller than the outer diameter of the second diaphragm is attached on the upper surface of the resin magnet of the electromagnetic electro-acoustic transducer of the first embodiment. As the magnetic resistance can be further reduced with this construction, a higher magnetic flux density can be obtained thereby making it possible to increase the compounding ratio of the soft magnetic material and enabling expansion of controllable ranges of the lowest resonant frequency and the sound pressure.
A sixth embodiment of the present invention is one in which the ratio of the soft magnetic material powder in the resin magnet of the electromagnetic electro-acoustic transducer of the first to the fifth embodiments is in the range 15 to 30% by weight and is capable of providing an transducer employing a superior resin magnet with a practical magnetic flux density and a high magnetic permeability.
A seventh embodiment of the present invention disposes a first diaphragm, a second diaphragm attached at the center of the first diaphragm and consisting of a magnetic material smaller than the first diaphragm, a center pole provided underneath the center of the second diaphragm with a magnetic gap in between, a coil wound around the outer periphery of the center pole, a ring-shaped magnet disposed on the outside of the coil, a yoke disposed in a manner such that it comes in contact with the lower parts of the center pole, and a ring-shaped magnetic plate having an inner diameter designed in a manner such that the magnetic flux from the second diaphragm enters generally vertically. This embodiment provides an electromagnetic electro-acoustic transducer in which a magnetic circuit is formed in a manner such that the magnetic flux of the magnet returns to the magnet through the yoke, center pole, second diaphragm and the magnetic plate, so that magnetic resistance is reduced by going through the magnetic plate, magnetic flux density is enhanced, and the attraction force toward the second diaphragm is almost maximized thus improving magnetic efficiency.